Various publications are cited herein, the disclosures of which are incorporated by reference in their entireties. However, the citation of any reference herein should not be construed as an admission that such reference is available as “Prior Art” to the instant application.
Cultivated agriculture has greatly increased efficiency of food production in the world. However, various insect pests have found it advantageous to seek out and exploit cultivated sources of food to their own advantage. These insect pests typically develop by a temporal sequence of events which are characteristic of their order. Many insects initially develop in a caterpillar or maggot-like larval form. Thereafter, they undergo a significant metamorphosis from which an adult emerges having characteristic anatomical features. Anatomic similarity is a reflection of developmental, physiological and biochemical similarities shared by these creatures. In particular, the principles of the insect ecdysteroid-hormone receptors and development, as described by Ashburner et al. (Cold Spring Harbor Symp. Quant. Biol. 38:655-662, 1974), likely would be shared by many different types of insects.
To prevent or reduce the destruction of cultivated crops by insects, organic molecules with pesticidal properties are used commonly in attempts to eliminate or reduce the insect populations. However, the ecological side effects of these pesticides, due in part to their broad activity and lack of specificity, and in part, to the fact that some of these pesticides are not easily biodegradable, significantly affect populations of both insect and other species of animals. Some of these organisms may be advantageous from an ecological or other perspective. Furthermore, as the insect populations evolve in directions to minimize the effects of the applied pesticides, the amounts of pesticides applied are often elevated so high as to cause significant effects on other animals, including humans, which are affected directly or indirectly by the application of the pesticides. Thus, an important need exists for both highly specific pesticides or highly active pesticides which have biological effects only on the species of animals targeted by the pesticides, and are biodegradable. Novel insect hormones which, like the ecdysteroids, act by complexing with insect members of the steroid receptor superfamily to control insect development, are likely candidates for pesticides with these desirable properties.
Growth, molting, and development in insects are regulated by the ecdysone steroid hormone (molting hormone) and the juvenile hormones (Dhadialla, et al., 1998, Amu. Rev. Entomol. 43: 545-569). The molecular target for ecdysone in insects consists of at least ecdysone receptor (EcR) and ultraspiracle protein (USP). EcR is a member of the nuclear steroid receptor super family that is characterized by signature DNA and ligand binding domains, and an activation domain (Koelle et al. 1991, Cell, 67:59-77). EcR receptors are responsive to a number of steroidal compounds such as ponasterone A and muristerone A. Recently, non-steroidal compounds with ecdysteroid agonist activity have been described, including the commercially available insecticides tebufenozide and methoxyfenozide (see International Patent Application No. PCT/EP96/00686 and U.S. Pat. No. 5,530,028). Both analogs have exceptional safety profiles to other organisms.
Polynucleotides encoding ecdysone receptors have been cloned from a variety of insect species, including Dipterans (see U.S. Pat. Nos. 5,514,578 and 6,245,531 B1), Lepidopterans, Orthopterans, Hemipterans, and one Homopteran Aphid, all from the class Arthropod. In particular, EcRs have been cloned from spruce budworm Choristoneura fumiferana EcR (“CfEcR”; Kothapalli et al., 1995 Dev Genet. 17: 319-30), a yellow meal worm Tenebrio molitor EcR (“TmEcR”; Mouillet et al., 1997, Eur. J. biochem. 248: 856-863), a tobacco hormworm Manduca sexta EcR (“MsEcR”; Fujiwara et al., 1995, Insect Biochem. Molec. Biol. 25, 845-856), a tobacco budworm Heliothies virescens EcR (“HvEcR”; Martinez et al., 1999, Insect Biochem Mol Biol. 29: 915-30), a golmidge Chironomus tentans EcR (“CtEcR”; Imhof et al., 1993, Insect Biochem. Molec. Biol. 23, 115-124), a silkworm Bombyx mori EcR (“BmEcR”; Swevers et al., 1995, Insect Biochem. Molec. Biol. 25, 857-866), a squinting bush brown Bicyclus anynana EcR (“BanEcR”), a buckeye Junonia coenia EcR (“JcEcR”), a fruit fly Drosophila melanogaster EcR (“DmEcR”; Koelle et al., 1991, Cell 67, 59-77), a yellow fever mosquito Aedes aegypti EcR (“AaEcR”; Cho et al., 1995, Insect Biochem. Molec. Biol. 25, 19-27), a blowfly Lucilia capitata (“LcEcR”), a sheep blowfly Lucilia cuprina EcR (“LucEcR”; Hannan and Hill, 1997, Insect Biochem. Molec. Biol. 27, 479-488), a blowfly Calliphora vicuna EcR (“CvEcR”), a Mediterranean fruit fly Ceratitis capitata EcR (“CcEcR”; Verras et al., 1999, Eur J Biochem. 265: 798-808), a locust Locusta migratoria EcR (“LmEcR”; Saleh et al., 1998, Mol Cell Endocrinol. 143: 91-9), an aphid Myzus persicae EcR (“MpEcR”; International Patent Application Publication WO99/36520), a fiddler crab Celuca pugilator EcR (“CpEcR”; Chung et al., 1998, Mol Cell Endocrinol 139: 209-27), and an ixodid tick Amblyomma americanum EcR (“AmaEcR”; Guo et al., 1997, Insect Biochem. Molec. Biol. 27: 945-962). The nucleotide and/or amino acid sequences of these ecdysone receptors have been determined and are publicly available.
The ecdysone receptor complex typically includes proteins that are members of the nuclear receptor superfamily wherein all members are generally characterized by the presence of an amino-terminal transactivation domain, a DNA binding domain (“DBD”), and a ligand binding domain (“LBD”) separated from the DBD by a hinge region. As used herein, the term “DNA binding domain” comprises a minimal polypeptide sequence of a DNA binding protein, up to the entire length of a DNA binding protein, so long as the DNA binding domain functions to associate with a particular response element. Members of the nuclear receptor superfamily are also characterized by the presence of four or five domains: A/B, C, D, E, and in some members F (see U.S. Pat. No. 4,981,784 and Evans, Science 240:889-895 (1988)). The “A/B” domain corresponds to the transactivation domain, “C” corresponds to the DNA binding domain, “D” corresponds to the hinge region, and “E” corresponds to the ligand binding domain. Some members of the family may also have another transactivation domain on the carboxy-terminal side of the LBD corresponding to “F”.
The DBD is characterized by the presence of two cysteine zinc fingers between which are two amino acid motifs, the P-box and the D-box, which confer specificity for ecdysone response elements. These domains may be either native, modified, or chimeras of different domains of heterologous receptor proteins. The EcR receptor, like a subset of the steroid receptor family, also possesses less well-defined regions responsible for heterodimerization properties. Because the domains of nuclear receptors are modular in nature, the LBD, DBD, and transactivation domains may be interchanged.
Gene switch systems are known that incorporate components from the ecdysone receptor complex. However, in these known systems, whenever EcR is used it is associated with native or modified DNA binding domains and transactivation domains on the same molecule. USP or RXR are typically used as silent partners. Applicants have previously shown that when DNA binding domains and transactivation domains are on the same molecule the background activity in the absence of ligand is high and that such activity is dramatically reduced when DNA binding domains and transactivation domains are on different molecules, that is, on each of two partners of a heterodimeric or homodimeric complex (see PCT/US01/09050).
The insect ecdysone receptor (EcR) heterodimerizes with Ultraspiracle (USP), the insect homologue of the mammalian RXR, and binds ecdysteroids and ecdysone receptor response elements and activates transcription of ecdysone responsive genes (Riddiford et al. 2000, Vitam Horm, 60: 1-73). The EcR/USP/ligand complexes play important roles during insect development and reproduction. The EcR is a member of the steroid hormone receptor superfamily and has five modular domains, A/B (transactivation), C (DNA binding, heterodimerization), D (Hinge, heterodimerization), E (ligand binding, heterodimerization and transactivation and in some cases, F (trans activation), domains. Some of these domains such as A/B, C and E retain their function when they are fused to other proteins.
Recently, ecdysone receptor based gene expression systems have been developed. Tightly regulated inducible gene expression systems or “gene switches” are useful for various applications such as gene therapy, large scale production of proteins in cells, cell based high throughput screening assays, functional genomics and regulation of traits in transgenic plants and animals. U.S. Pat. No. 6,265,173 B1 discloses that various members of the steroid/thyroid superfamily of receptors can combine with Drosophila melanogaster ultraspiracle receptor (USP) or fragments thereof comprising at least the dimerization domain of USP for use in a gene expression system. U.S. Pat. No. 5,880,333 discloses a Drosophila melanogaster EcR and ultraspiracle (USP) heterodimer system used in plants in which the transactivation domain and the DNA binding domain are positioned on two different hybrid proteins.
The first version of an EcR-based gene switch used Drosophila melanogaster EcR (DmEcR) and Mus musculus RXR (MmRXR) and showed that these receptors in the presence of steroid, ponasterone A, transactivate reporter genes in mammalian cell lines and transgenic mice (Christopherson et al. 1992, PNAS 89:6314-6318; No et al. 1996, PNAS 93:3346-3351). Later, Suhr et al. (1998, Proc. Natl. Acad. Sci. U.S.A. 95: 7999-8004) showed that non-steroidal ecdysone agonist, tebufenozide, induced high level of transactivation of reporter genes in mammalian cells through Bombyx mori EcR (BmEcR) in the absence of exogenous heterodimer partner.
International Patent Applications Nos. PCT/US97/05330 (WO 97/38117) and PCT/US99/08381 (WO99/58155) disclose methods for modulating the expression of an exogenous gene in which a DNA construct comprising the exogenous gene and an ecdysone response element is activated by a second DNA construct comprising an ecdysone receptor that, in the presence of a ligand therefor, and optionally in the presence of a receptor capable of acting as a silent partner, binds to the ecdysone response element to induce gene expression. The ecdysone receptor of choice was isolated from Drosophila melanogaster. Typically, such systems require the presence of the silent partner, preferably retinoid X receptor (RXR), in order to provide optimum activation. In mammalian cells, insect ecdysone receptor (EcR) heterodimerizes with retinoid X receptor (RXR) and regulates expression of target genes in a ligand dependent manner. International Patent Application No. PCT/US98/14215 (WO 99/02683) discloses that the ecdysone receptor isolated from the silk moth Bombyx mori is functional in mammalian systems without the need for an exogenous dimer partner.
Unfortunately, these USP-based systems are constitutive in animal cells and therefore, are not effective for regulating reporter gene expression. Drawbacks of the above described EcR-based gene regulation systems include a considerable background activity in the absence of ligands and non-applicability of these systems for use in both plants and animals (see U.S. Pat. No. 5,880,333).
Recently, an improved ecdysone receptor-based inducible gene expression system has been developed in which the transactivation and DNA binding domains are separated from each other by placing them on two different proteins results in greatly reduced background activity in the absence of a ligand and significantly increased activity over background in the presence of a ligand (pending application PCT/US01/09050, incorporated herein in its entirety by reference). This two-hybrid system is a significantly improved inducible gene expression modulation system compared to the two systems disclosed in applications PCT/US97/05330 and PCT/US98/14215. The two-hybrid system exploits the ability of a pair of interacting proteins to bring the transcription activation domain into a more favorable position relative to the DNA binding domain such that when the DNA binding domain binds to the DNA binding site on the gene, the transactivation domain more effectively activates the promoter (see, for example, U.S. Pat. No. 5,283,173). Briefly, the two-hybrid gene expression system comprises two gene expression cassettes; the first encoding a DNA binding domain fused to a nuclear receptor polypeptide, and the second encoding a transactivation domain fused to a different nuclear receptor polypeptide. In the presence of ligand, the interaction of the first polypeptide with the second polypeptide effectively tethers the DNA binding domain to the transactivation domain. Since the DNA binding and transactivation domains reside on two different molecules, the background activity in the absence of ligand is greatly reduced.
A two-hybrid system also provides improved sensitivity to non-steroidal ligands for example, diacylhydrazines, when compared to steroidal ligands for example, ponasterone A (“PonA”) or muristerone A (“MurA”). That is, when compared to steroids, the non-steroidal ligands provide higher activity at a lower concentration. In addition, since transactivation based on EcR gene switches is often cell-line dependent, it is easier to tailor switching systems to obtain maximum transactivation capability for each application. Furthermore, the two-hybrid system avoids some side effects due to overexpression of RXR that often occur when unmodified RXR is used as a switching partner. In a preferred two-hybrid system, native DNA binding and transactivation domains of EcR or RXR are eliminated and as a result, these hybrid molecules have less chance of interacting with other steroid hormone receptors present in the cell resulting in reduced side effects.
Applicants have now obtained and determined the full-length coding sequence of an additional homopteran EcR polynucleotide from whitefly for use in methods of modulating gene expression in a host cell and methods of identifying molecules that modulate activity of whitefly EcR. As described herein, Applicants' invention provides novel whitefly ecdysone receptor polypeptides and novel polynucleotides encoding these polypeptides that are useful as components of gene expression systems for highly specific regulation of recombinant proteins in host cells or in methods for identifying new molecules which may act as agonists or antagonists of a homopteran insect ecdysone receptor.